I try to never sew over pins because of all the possibilities of accidents happening. It can severely damage a machine-especially a computerized machine. The needle can break in more than one place and send a shard into the bobbin area and cause really expensive repair. Most significant is the possibility of flying up into your eye. So please, stitch up to the pin but not over it. If you have stitched the sections of your blocks accurately they will nest easily together. If you think you must have the pins for accuracy you are generally trying to 'make' something fit by distorting it by stretching or working in extra fullness. In either case the distortion will remain after taking out the pins. Be safe!